1. Field of the Invention
This invention relates to a method and apparatus for splicing light transmitting fibers such as, for example, optical waveguides.
High capacity communication systems operating around 10.sup.15 Hz are needed to accommodate future increases in communication traffic. Optical waveguides, which are the most promising medium for transmission at such frequencies, normally consist of an optical fiber having a transparent core surrounded by transparent cladding material. Very low loss glass optical fibers have been produced in accordance with the methods disclosed in U.S. Pat. Nos. 3,775,075 and 3,711,262 issued to D. B. Keck et al. Because of the low losses of such fibers, light can be propagated therethrough many kilometers without the need for repeaters. When the length between repeaters exceeds the length which can be drawn, a plurality of fibers must be connected to provide the desired length. Also, a low loss method is needed for connecting broken fibers. A permanent splice is more desirable than a simple connector in these instances. A splice avoids the inherent Fresnel loss experienced in connectors and does not require bulky fiber supporting structure associated with connectors wherein the endfaces of two adjacent fibers are merely secured together in light transmitting alignment.
2. Description of the Prior Art
Permanent optical fiber connections have been made by applying a transparent bonding material to the fiber ends while they are held in axial alignment. A chemically set glue typically requires several hours to cure. A thermoplastic bonding material has also been employed. Fibers are aligned colinearly and a quantity of low melting point transparent thermoplastic is inserted at the junction thereof. Heat is applied to melt the thermoplastic causing it to flow around the fiber ends. A sleeve has been employed with both chemically cured and thermoplastic bonding materials, the bonding material being introduced through a hole in the sleeve after the fibers have been inserted. Even though attempts are made to match the refractive index of the bonding material to that of the fiber core material, the slight discrepancy which invariably occurs introduces losses due to reflection. Moreover, the sleeve which adds strength to the fiber junction and assists in the axial alignment of the fibers causes the splice to be undesirably bulky. When using such sleeves, the tolerance between the fiber and the inner surface of the sleeve must be very close, i.e., about 1.mu.m, to keep the fibers properly aligned, and such a tolerance is difficult to obtain. Furthermore, bubbles can form at the fiber junction inside sleeve joints, thereby rendering them useless. When epoxy is employed alone at the fiber joint, the resultant joint is too weak to keep the fiber ends aligned when transverse pressure is applied.
Fibers consisting of low melting point material have been thermally fused, end to end, to provide a good mechanical joint having low transmission loss. In accordance with this technique, the fiber ends are aligned, leaving a space therebetween for thermal expansion. Current is passed through a resistance wire surrounding the fiber ends causing the wire to heat up and fuse the ends together. When the fiber is heated, the longitudinal expansion thereof closes the gap left between the fiber ends. Because of the length of fiber which is heated by the loop of resistance heating wire the effect of gravity causes poor alignment of the cores of the fibers unless they are vertically disposed during the splicing process. Good mechanical joints can be made having transmission losses as low as about 0.5 dB on fibers of glass having a softening temperature of about 700.degree.C. However, fibers of high silica content glass such as those described in the aforementioned Keck et al. patents, cannot be joined by this technique, since temperatures up to about 1600.degree.C. are required. Moreover, when the hot wire butt-fusion method is employed to splice single mode waveguides, losses are much greater than for splicing multimode waveguides, maximum coupling efficiencies of only 70% being reported and reproducibility being poor.